Good Evening, PKM-25,

Sounds about right for the 50mm lens, although the format shape might give you a little more than 20" in the long dimension. For 6 x 6, a 75mm or 80mm lens should go a little beyond 20 x 20. I've never tried a 90mm with that size negative, but I imagine that 20 x 20 would be about the most you could get.

As to your missing negative size scale, there is a previous thread here on APUG on which I posted the markings for different negative sizes; I can't remember exactly what they are right now, but being a little off, as long as the light covers a given negative, won't make a lot of difference.